Monitoring system for sound recording



Feb'l, w44. J. LpFl'ELDs 2,34%706 MONITORING SYSTEM FOR SOUND RECORDING Filed May 31, 1940 1a, l MIXER Po-r pgs IWW/* I AMPLIFIER Patented Feb. 15,1944

MONITORING SYSTEM FOR SOUND RECORDING j James L. Fields, North Hollywood, Gau-assigner to Badi of Delaware o Corporation of America, a corporation Application May 31, 1940, serial No. 338,033

. 3 claims. (01.171795) This invention relates to sound recording sys- I tems and particularly to apparatus for monitoring the volume or level of sound being impressed upon a recording medium, such as a motion pic-v ture lm.

In the production of sound motion pictures, the sound is usually recorded simultaneously with the dphotographing of the picture scene, the picture camera being located at the scene of action, and the sound recorder being located at a distant point.` Of course, a portion of the sound recordto be characteristic of this invention are pointed ing apparatus is also located at the scene of action, this portion usually consisting of the pick--` 4 up microphone, a pre-amplifier, a mixer pot" or potentiometer, a volume indicator in the form of a meter for visually monitoring the signal level,

and a set of headphones for audibly monitoring the signal. The remainder of therecordingapparatus is located either at a distant recording building or in a mobile recording unit.

In setting up a recording. system so that the respective gains of the various amplifiers between the microphone and recorder are properly adjusted with respect to the signal input level at the microphone and the final level at the recorder, it is necessary to make several tests at various points inthe transmission circuit. It is also necessary that the monitoring elements, such as the headphones and meter, be adjusted with respect to the'range of amplitudes being recorded and impressed upon the illm. Adjustments of amplitude levels are observed on volume indicator meters of the swinging needle type .employing rectier units. It has been found, however, that the present type of volume indicator or VI meters, although satisfactory for steady currentconditions, are difficult to -read when the volume levels vary rapidly, such as in dramatic dialogue and music. The readabilityof such a VI meter is particularly important in the control of signal levels especially when the signal range exceeds that of the nlm range? The presentinvention provides a visual typel ofmonitor meter which has been found to provide the mixing operator with an accurate indication of the actual vlight variations being impressed upon the film at all levels as well as a meter which is easily readable during rapid volume variations.

The principal object of .the invention, therefore, is to facilitate the monitoring of a sound recording system.

Another object of the invention is to improve the readability of a vol e indicator meter. A further object of he invention is to provide provides` a faster and more stable actionfor variations involume impressed thereon.

Although the novel features which are believed out with particularity in the claims appended herewith, the manner of its organization and the mode of `its operation will be better understood by referring to the following .description read' in commotion with the accompanying drawing forming apart thereof in which Fig. 1 is a diagrammatic drawing of the usual type of sound recording system emphasizing the monitoring elements embodying the invention; and i Fig. 2 is a schematic drawing of the circuit for the volume indicator'neter of Fig. l.

Referring now 'to the drawing, the vsound recorder is diagrammatically represented `solely by a light-source 5, a galvanometer mirror 6 with its actuating coil 1, and alm 8, the light monitoring portion thereof being shown .simply by mirrors I 0 and II and a monitor card I2 withits light image I3 for both the modulation and noise re 177,907, filed December 3, '1937, now Patent No.

Located on the stage or at the scene of action is a microphone vI 5 with its pre-amplifier I6 feeding .an attenuator or variable potentiometer I1 .called a mixer pot. The outputs 'of several microphones positioned around a set may be mixed at II, only one being shown for. the salie of clarity. 'I'he output of the mixer I1 is transmitted to an amplifier I9 and then to a second amplifier 20 which feeds the light modulating galvanometer coil 1. Bridged across the output of amplifier 20 is a monitor amplifier 22 which feeds a loudspeaker monitor 23 'positioned at the recorder, a pair ofl headphones 24 located at the scene of action, and an attenuator 25, the output of the attenuator being connected to a volume indicator meter 21, also at the scene of action. For test purposes, there is shown in dotted lines,

. bridged across the output of amplifier I9, a volume indicator 30.

The above description includes the essential av timing circuit` for a volume indicator which o5 building at a distant point. 'Ifhus, although the less of their frequency and amplitude.

recordist is able to initially adjust the signal vstrength and is aware of the amplitude thereof impressed on the lm by his monitoring card I2, the mixer on the stage actually controls the volume level during recording. Thus, the mixer must have a volume indicator which is particularly adapted tov provide an accurate and readable indication of the sound level variations regard- The present type of meter nowknown does not provide the necessary accuracy and readability, particularly during volume level variations accompanying the recording of dramatic dialogue and music.

In setting up the apparatus. the volume in'dicator 30 is bridged across the output oi amplier I9 or input of amplifier 20. Then with a tone impressed on the microphone l5, the input to the ampliileriS is adjusted so that the meter 30 reads zero level, which corresponds to 1.73 volts for a 5,00 ohm line. After this adjustment has been made, the amplier I9 is then backed down in gain by 10 db. At this point. the meter 21 is adjusted by the variable attenuator 25 so that the meter 21 reads +4 db.` This is the normal setup.

After thisA adjustment is made, thevrecordist makes the necessary adjustments to his galvanometer 'mirror 6 and other recording elements.

Now, during recording, the mixer varies the input level to amplifier I9 by the'mixer pot I1 and is guided by his observation of the needle 28 of meter 21. Although the headphones 24 give some indication of volume level, they are usually lused to determine the quality of the signal and to an impedance of 525 ohms at 1000 cycles is as follows: The value of Ri is 3735 ohms; Rz, 1950 ohms; and Rg 1250 ohms. For other coil impedances, different resistance values are necessary to provide the desired needle action. All these elements may be enclosed within the meter casing.

The needle 28 of meter 21 rapidly follows the voltage increases but has aslow decay period, this period being such that for dramatic variations in volume of the signal, the needle will not follow the valleys of the variations and will, therefore, provide a more steady action. This action provides greater readability of the meter at all volume levels andparticularly for low volumes in the neighborhood of .10 db to -5 db, since the average position of the needle is above that of the average variation of the signal being recorded. These meters are calibrated in decibels, or db, which is a logarithmic scale, and the scale dimensions are smaller at the-low volume levels, as shown in meter 21. It has been found in actual practice that the use of the above-described timing circuit has improved the quality of the recorded sound to a, considerable extent, improvement being particularly noticeable in recordings of signal ranges considerably beyond the film range. This is due, no doubt, to the mixers ability to control peak-clipping of the high mmentary peaks, while maintaining the low volume levels above the lower film limit.

will follow all variations in volume of the signal.

The meters now known in the art for this use are slow to respond to short, momentary impulses and are particularly insensitive at low volume levels and they follow the peaks and valleys of the volume variations so that the mixing operator is ductors and I6 are connected to the attenuator or to the monitor 22, in case no attenuator is used. The o ther ends of conductors and 36 are connected to a bridge type rectifier unit 31 composed of 38, Il, l0 and 4I. The vdirect current output from the rectier unit 31is connected to a moving coil. of the meter 21 -in series with resistances R1 and Rs. Shunted across the moving coil is a resistance R2, andv shunted across the output lof the rectifier 31 is a capacitor C. The values of these elements are such as to provide a response which is particu-A larly sensitive to voltage increases both large and small and which has a slow, restoration period 'caused,by thefdischarge of condenser C. One set of values f'o'r a meter having a moving coil with Referring now to Fig. 2, the open ends 'of con- 5' What I claim as my invention is: l

1. A meter system for measuring current levels, said system comprising a moving coil, means connected to said coil for impressing current thereon, a rectifier, a resistance in series with said vrectifier and moving coil, a resistance in shunt f with said coil and a condenser in shunt with said rectifier, said coil having an impedance .of substantially 525 ohms at 1000 cycles, said series resistance having a value ofA substantially 5000 ohms, said shunt resistance a value of substan- .ations thereon; arectitler, a resistancev in series with said rectiiier and moving coil, a resistance in shunt with said coil, and a condenser in shunt with said rectiiier, said series resistance being divided into two sections connected in each lead to said coil, one of said sections having a value of.

approximately 3750 ohms, and the other section having a value of approximately 1250 ohms.

3. A volume indicating meter comprising a moving coil, a needle adapted to be moved with movement of said coil, a scale over which said and a resistance of approximately 5000 ohms in series with said rectiiier and said coil, said coil having anlmpedance of approximately 525 ohmsl at 1000 cycles.

JAMES LFIELDS. 

